Displays are used on notebook PCs, tablets, mobile devices, televisions, and other electronic devices. Like most electronic devices, displays must be calibrated to accurately display video and graphic images. For example, the common voltage of a display is calibrated for optimum viewing and operation. Without proper calibration, the image on the display can substantially flicker. In some types of displays, such as liquid crystal displays (LCDs), e-ink displays, and electro-wetting displays, the pixel material can be damaged if the common voltage is not set correctly.
Some displays are characterized by a common voltage (VCOM), herein referred to as VCOM displays. The VCOM voltage is applied to a common voltage reference plane, referred to as the VCOM reference plane, of a VCOM display panel. The VCOM reference plane distributes the VCOM voltage to each pixel in the VCOM display panel. Application of the VCOM voltage allows for adjustment of the absolute voltage applied to the pixel, thereby turning the pixel on and off. Proper calibration of the VCOM voltage enables correct operation of each pixel and also maintains a substantially zero volt average across the pixel which prevents the pixel material from becoming damaged, such as causing an image to be burned into the display screen.
The VCOM voltage is supplied using one or more appropriate VCOM application circuits. Conventional VCOM application circuits use a Class AB amplifier to generate the proper VCOM voltage level that is provided to the VCOM display panel. FIG. 1A illustrates an exemplary conventional VCOM application circuit 10. A digital-to-analog converter (DAC) 2 receives as input a digital code representative of the proper VCOM voltage level. The DAC 2 outputs a converted analog signal to a first input of an amplifier 4. The amplifier 4 is a Class AB operational amplifier. A second input of the amplifier 4 is a feedback signal. The amplifier 4 is supplied with an analog power supply voltage AVDD. An output of the amplifier 4 is the VCOM voltage level that is supplied to the VCOM reference plane of a LCD panel 20. The VCOM reference plane can be modeled as a distributed RC. In some applications, the VCOM voltage level is substantially constant. An alternative configuration of the VCOM application circuit 10′, as shown in FIG. 1B, can also be implemented to provide a constant VCOM voltage level. The VCOM application circuit 10′ includes a local feedback from the output of the Class AB amplifier 4′ to the second input of the Class AB amplifier 4′. The Class AB amplifier 4′ can be the same or different than the Class AB amplifier 4 in FIG. 1A. In other applications, the VCOM voltage level can be adjusted using the VCOM application circuit 10 (FIG. 1A) by providing a feedback signal from the VCOM plane 20 to the second input of the Class AB amplifier 4.
In many applications, the VCOM amplifier drives a point on one side of the VCOM reference plane, and receives a feedback voltage from the other side of the VCOM reference plane. Since the VCOM reference plane has a relatively large resistance, it is difficult to control the absolute voltage across the entire VCOM reference plane, which is necessary to properly operating each pixel. Further, when the pixels are refreshed, turned on, or turned off, there is a resulting change in applied pixel voltage, which capacitively couples current into the VCOM reference plane. As such, the localized voltages in the VCOM reference plane are changing as different pixels are updated, further effecting the absolute voltage across the entire VCOM reference plane. The feedback voltage, such as voltage VCOM_FB in FIG. 1A, is input to the VCOM amplifier to adjust the driving VCOM voltage. This provides an active feedback for providing an average voltage across the VCOM reference plane. However, adjusting the VCOM voltage in response to the feedback voltage VCOM FB results in large current outputs due to the large load capacitance of the VCOM reference plane. These large currents cause severe heat rise in the linear VCOM amplifier.